Display apparatus, method of driving a display, and electronic device

ABSTRACT

In a display apparatus including a switching transistor, a correction voltage for eliminating an effect of a variation in a characteristic of a driving transistor is stored in a storage capacitor. The switching transistor is disposed between one current terminal of the driving transistor and a light emitting element. The switching transistor turns off during the non-light emission period thereby to electrically disconnect the light emitting element from the one current terminal of the driving transistor thereby preventing a leakage current from flowing through the light emitting element during the period in which the correction unit operates, and thus preventing the correction voltage from having an error due to the leakage current.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a Continuation application of U.S. patent application Ser. No.13/354,908, filed Jan. 20, 2012, which is a Continuation application ofU.S. patent application Ser. No. 13/137,092, filed Jul. 20, 2011, whichis a Continuation application of U.S. patent application Ser. No.12/927,000, filed Dec. 22, 2010, now U.S. Pat. No. 8,026,874, issuedSep. 27, 2011, which is a Continuation application of U.S. patentapplication Ser. No. 12/071,284, filed Feb. 19, 2008, now U.S. Pat. No.7,880,695, issued Feb. 1, 2011, which in turn claims priority fromJapanese Patent Application JP 2007-041196, filed in the Japanese PatentOffice on Feb. 21, 2007, the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display apparatus including an arrayof pixels each including a light emitting element, and moreparticularly, to an active matrix display apparatus adapted to controlcurrents flowing through light emitting elements, for example, of theorganic electroluminescent type by using insulating gate field effecttransistors disposed in the respective pixels. The present inventionalso relates to a method of driving such a display apparatus and to anelectronic device using such a display apparatus.

2. Description of the Related Art

In an image display apparatus such as a liquid crystal display, a largenumber of liquid crystal pixels are arranged in the form of a matrix andthe intensity of light transmitting through or reflected from therespective pixels are controlled in accordance with image informationthereby displaying an image. Organic electroluminescent elements may beused as light emitting elements disposed in the respective pixels. Incontrast to liquid crystal pixels which are not of the self-luminoustype, the organic electroluminescent display is of the self-luminoustype and has the following advantages. That is, the organicelectroluminescent display provides high visibility compared with theliquid crystal display, does not need backlighting, and can respond at ahigh speed. The intensity levels (gray-scale levels) of each lightemitting element can be controlled by controlling the current flowingthrough the light emitting element. That is, the organicelectroluminescent display device is of the current-controlled type. Inthis respect, the organic electroluminescent device is very differentfrom the liquid crystal display which is of the voltage-controlled type.

As with the liquid crystal display, the organic electroluminescentdisplay can be driven by a passive matrix addressing scheme or an activematrix addressing scheme. Although the passive matrix address schemeneeds a simpler configuration, the disadvantage of this scheme is thatit is difficult to realize a large-size high-resolution display. For theabove reason, the active matrix address scheme is currently receivingmore attention. In this scheme, the current flowing through the lightemitting element disposed in each pixel is controlled by an activeelement (for example, a thin-film transistor (TFT)) disposed in eachpixel. Further detailed descriptions of this scheme and associatedtechniques may be found, for example, in patent documents listed below.

-   Japanese Unexamined Patent Application Publication No. 2003-255856-   Japanese Unexamined Patent Application Publication No. 2003-271095-   Japanese Unexamined Patent Application Publication No. 2004-133240-   Japanese Unexamined Patent Application Publication No. 2004-029791-   Japanese Unexamined Patent Application Publication No. 2004-093682-   Japanese Unexamined Patent Application Publication No. 2006-215213

SUMMARY OF THE INVENTION

For example, pixel circuits are disposed at respective intersections ofscanning lines in the form of rows for supplying control signals andsignal lines in the form of columns for supplying an image signal, andeach pixel circuit includes, at least, a sampling transistor, a storagecapacitor, a driving transistor, and a light emitting element. Thesampling transistor turns on in response to the control signal suppliedvia the scanning line and samples the image signal supplied via thesignal line. The storage capacitor stores an input voltage (signalvoltage) corresponding to the sampled image signal. The drivingtransistor supplies an output current during a predetermined lightemission period in accordance with the input voltage stored in thestorage capacitor. In general, the output current depends on the carriermobility of the channel region of the driving transistor and alsodepends on the threshold voltage. The light emitting element is drivenby the output current of the driving transistor so as to emit light withan intensity corresponding to the image signal.

The input voltage applied to the gate from the storage capacitor causesan output current to flow between the source and drain of the drivingtransistor, and the output current is supplied to the light emittingelement. In general, the intensity of light emitted by the lightemitting element varies in proportion to the current flowingtherethrough. The output current of the driving transistor is controlledby the gate voltage given by the input voltage written in the storagecapacitor. In the pixel circuit according to the related technique, thecurrent supplied to the light emitting element is controlled by changingthe input voltage applied to the gate of the driving transistor inaccordance with the input image signal.

The operating characteristic of the driving transistor can be expressedby a characteristic equation shown below.Ids=(1/2)WW/L)Cox(Vgs−Vth)²In the transistor characteristic equation shown above, Ids is the draincurrent flowing between the source and drain, which is supplied, as theoutput current, to the light emitting element in the pixel circuit. Vgsis the gate voltage applied to the gate with respect to the source. Inthe pixel circuit, Vgs is given as the input voltage as described above.Vth is the threshold voltage of the transistor, and μ is the mobility ofa semiconductor thin film forming the channel of the transistor. W, L,and Cox denote the channel width, the channel length, and the gatecapacitance. As can be seen from the transistor characteristic equation,when the thin film transistor operates in the saturation region, if thegate voltage Vgs increases beyond the threshold voltage Vth, thetransistor turns on and a drain current Ids flows. Theoretically, as canbe seen from the transistor characteristic equation, if the gate voltageVgs is constant, a constant drain current Ids is supplied to the lightemitting element. Therefore, ideally, if an image signal with an equalsignal level is supplied to all pixels of a screen, all pixels will emitlight with an equal intensity, and thus a perfect uniformity inbrightness will be obtained over the whole screen area.

However, in practice, thin film transistors (TFTs) formed using asemiconductor thin film, such as a polysilicon thin film, vary in devicecharacteristics from one transistor to another. In particular, thethreshold voltage Vth varies from pixel to pixel. As can be seen fromthe transistor characteristic equation described above, the variation inthe threshold voltage Vth of driving transistors leads to a variation inthe drain current Ids even if the gate voltage Vgs is equal, and thus avariation occurs in brightness from one pixel to another. This makes itdifficult to achieve perfect uniformity in brightness over the screen.In view of the above, it is known to configure the pixel circuit to havea function of eliminating effects of the variation in the thresholdvoltage of the driving transistor (for example, see Japanese UnexaminedPatent Application Publication No. 2004-133240).

By configuring the pixel circuit so as to have the function ofeliminating the effects of the variation in the threshold voltage, it ispossible to improve the uniformity in brightness over the screen.However, of various parameters of the characteristics of the polysiliconthin film transistor, not only the threshold voltage but also themobility μ varies from one transistor to another. As can be seen fromthe transistor characteristic equation described above, the variation inmobility μ can lead to a variation in the drain current Ids even if thegate voltage Vgs is equal. As a result, the intensity of emitted lightvaries from one pixel to another, and thus degradation occurs inuniformity of brightness over the screen. There is a technique toconfigure the pixel circuit so as to have a function of eliminating theeffects of the variation in mobility in addition to that in thresholdvoltage of the driving transistor (for example, see Japanese UnexaminedPatent Application Publication No. 2006-215213).

The threshold voltage correction mechanism or the mobility correctionmechanism implemented in each pixel circuit generally operates toperform a predetermined correction process during a period (non-lightemission period) in which no light is emitted by the light emittingelement. Even in the non-light emission period, a leakage current canoccur in the light emitting element. The leakage current varies from onelight emitting element disposed in a pixel to another. Because theleakage current occurs in the no-light emission period, the leakagecurrent has an adverse effect on the accuracy of the correctionoperation. In particular, the variation in the leakage current of thelight emitting element from one pixel to another leads to a variation inaccuracy of the correction operation, and thus to a variation in theintensity of emitted light from pixel to pixel. As a result, degradationoccurs in uniformity of brightness over the screen.

SUMMARY OF THE INVENTION

In view of the above, it is desirable to provide a display apparatuscapable of accurately correcting light intensity regardless of leakagecurrents of light emitting elements.

According to one embodiment, a display apparatus includes a switchingtransistor, and a correction voltage for eliminating an effect of avariation in a characteristic of a driving transistor is stored in astorage capacitor. The switching transistor is disposed between onecurrent terminal of the driving transistor and a light emitting element.The switching transistor turns off during the non-light emission periodthereby to electrically disconnect the light emitting element from theone current terminal of the driving transistor thereby preventing aleakage current from flowing through the light emitting element duringthe period in which the correction unit operates, and thus preventingthe correction voltage from having an error due to the leakage current.

According to another embodiment, there is provided a display apparatusincluding a pixel array and a driving unit adapted to drive the pixelarray, the pixel array including scanning lines disposed in the form ofrows, signal lines disposed in the form of columns, and pixels disposed,in the form of an array, at respective intersections of the scanninglines and the signal lines, each pixel including at least a samplingtransistor, a driving transistor, a light emitting element, and astorage capacitor, a control terminal of the sampling transistor beingconnected to a corresponding scanning line, one of two current terminalsof the sampling transistor being connected to a corresponding signalline and the control terminal of the driving transistor, one of twocurrent terminals of the driving transistor being connected to the lightemitting element and the other one being connected to a power supplyline, the storage capacitor being connected between the control terminaland one of the two current terminals of the driving transistor, thedriving unit being configured to output a control signal over a selectedscanning line to turn on the sampling transistor connected to theselected scanning line, and output an image signal over a selectedsignal line to write the image signal in the storage capacitor via theturned-on sampling transistor, whereby the driving transistor supplies adriving current, depending on a signal voltage of the written imagesignal, to the light emitting element during a predetermined lightemission period, while the driving transistor supplies no drivingcurrent during a non-light emission period, each pixel includingcorrection means and a switching transistor, the correction means beingadapted to operate in the non-light emission period so as to write acorrection voltage in the storage capacitor thereby to eliminate aneffect of a variation in a characteristic of the driving transistor, theswitching transistor being disposed between one of the two currentterminals of the driving transistor and the light emitting element, andthe switching transistor being adapted to turn off during the non-lightemission period thereby to electrically disconnect the light emittingelement from the one of the two current terminals of the drivingtransistor thereby preventing a leakage current from flowing through thelight emitting element during the period in which the correction meansoperates and thus preventing the correction voltage from having an errordue to the leakage current.

The correction means may additively write a correction voltagecorresponding to a threshold voltage of the driving transistor into thestorage capacitor so as to eliminate an effect of a variation in thethreshold voltage of the driving transistor. The correction means maysubtract a correction voltage depending on a mobility of the drivingtransistor from the signal voltage of the image signal written in thestorage capacitor so as to eliminate an effect of a variation in themobility of the driving transistor.

The present invention provides great advantages as described below. Thatis, in the display apparatus according to the embodiment of the presentinvention, a switching transistor is disposed between the source servingas a current output terminal of a driving transistor and the anode of alight emitting element. During the non-light emission period, theswitching transistor is turned off to electrically disconnect the anodeof the light emitting element from the source of the driving transistor.In this period, the correction means operates such that a correctionvoltage is written into the storage capacitor from the current outputterminal (source) of the driving transistor so as to eliminate theeffects of the variation in characteristic of the driving transistor.Because the anode of the light emitting element is electricallydisconnected from the source of the driving transistor during thenon-light emission period, no leakage current flows through the lightemitting element and thus the potential of the current output terminalof the driving transistor does not have an error due to the leakagecurrent. This makes it possible for the display apparatus to accuratelymake the correction without being influenced by leakage currents oflight emitting elements, and thus it is possible to improve theuniformity in brightness over the screen.

In the correction process, it is necessary to apply the signal potentialfrom the signal line to the control terminal (gate) of the drivingtransistor even in the non-light emission period. The cathode of thelight emitting element is maintained at a predetermined fixed cathodepotential. In the case where the anode of the light emitting element isconnected to the source of the driving transistor during the correctionprocess as in the related technique, it is necessary to adjust therelative signal potential with respect to the cathode potential toachieve high accuracy in the correction process. In contrast, in theembodiment of the invention, because the anode of the light emittingelement is electrically disconnected from the source of the drivingtransistor during the correction process, it is allowed to set thesignal potential applied to the gate of the driving transistor and thecathode potential of the cathode of the light emitting elementindependently of each other. Thus, the signal potential and the cathodepotential can be properly set so as to achieve low power consumption ofthe panel and/or low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an overall configuration of afirst comparative example of a display apparatus;

FIG. 2 is a circuit diagram illustrating a specific configuration of thefirst comparative example of the display apparatus;

FIG. 3 is a timing chart provided for an explanation of an operation ofthe first comparative example of the display apparatus;

FIG. 4 is a graph illustrating a current-voltage characteristic of alight emitting element;

FIG. 5 is circuit diagram illustrating a display apparatus according afirst embodiment of the present invention;

FIG. 6 is timing chart provided for an explanation of an operation ofthe display apparatus according to the first embodiment of the presentinvention;

FIG. 7 is a circuit diagram of a second comparative example of a displayapparatus;

FIG. 8 is a timing chart provided for an explanation of an operation ofthe second comparative example of the display apparatus;

FIG. 9 is circuit diagram illustrating a display apparatus according asecond embodiment of the present invention;

FIG. 10 is timing chart provided for an explanation of an operation ofthe display apparatus according to the second embodiment of the presentinvention;

FIG. 11 is a circuit diagram of a third comparative example of a displayapparatus;

FIG. 12 is a timing chart provided for an explanation of an operation ofthe third comparative example of the display apparatus;

FIG. 13 is circuit diagram illustrating a display apparatus according athird embodiment of the present invention;

FIG. 14 is timing chart provided for an explanation of an operation ofthe display apparatus according to the third embodiment of the presentinvention;

FIG. 15 is a circuit diagram of a fourth comparative example of adisplay apparatus;

FIG. 16 is a timing chart provided for an explanation of an operation ofthe fourth comparative example of the display apparatus;

FIG. 17 is circuit diagram illustrating a display apparatus according afourth embodiment of the present invention;

FIG. 18 is timing chart provided for an explanation of an operation ofthe display apparatus according to the fourth embodiment of the presentinvention;

FIG. 19 is a cross-sectional view showing a device structure of adisplay apparatus according to an embodiment of the present invention;

FIG. 20 is a plan view illustrating a module structure of the displayapparatus according to an embodiment of the present invention;

FIG. 21 is a perspective view of a television set using a displayapparatus according to an embodiment of the present invention;

FIG. 22 is a perspective view of a digital still camera using a displayapparatus according to an embodiment of the present invention;

FIG. 23 is a perspective view of a notebook personal computer using adisplay apparatus according to an embodiment of the present invention;

FIG. 24 is a schematic diagram of a portable terminal using a displayapparatus according to an embodiment of the present invention; and

FIG. 25 is a perspective view of a video camera using a displayapparatus according to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described in further detail below withreference to embodiments in conjunction with the accompanying drawings.First, to illustrate a background of the invention, a first comparativeexample of an active matrix display apparatus having a threshold voltagecorrection function and a mobility correction function is describedbelow referring to FIG. 1. In this first comparative example, each pixelis basically composed of five transistors, one capacitor, and one lightemitting element. As shown in FIG. 1, in the first comparative example,the active matrix display apparatus is basically composed of a pixelarray 1 and a driving unit disposed in a peripheral area. The drivingunit includes a horizontal selector 3, a write scanner 4, a drivescanner 5, a first correction scanner 71, and a second correctionscanner 72. The pixel array 1 includes scanning lines WS extending in arow direction, signal lines SL extending in a column direction, andpixels 2 disposed in an array form at respective intersections of thescanning lines WS and the signal lines SL. To realize the capability ofdisplaying a color image, three types of pixels may be used so that oneof three primary colors (for example, red (R), green (G), and blue (B))is represented by one of the three types of pixels, although the pixelarray does not necessarily need to be configured in such a manner. Thesignal lines SL are driven by the horizontal selector 3. The horizontalselector 3 supplies an image signal over the signal lines SL. Thescanning lines WS are scanned by the write scanner 4. Additionalscanning lines DS, AZ1 and AZ2 are disposed such that they extendparallel to the scanning lines WS. The scanning lines DS are scanned bythe drive scanner 5. The scanning lines AZ1 are scanned by thecorrection scanner 71. The scanning lines AZ2 are scanned by the secondcorrection scanner 72. When each pixel 2 is selected by a scanning lineWS, the pixel 2 samples the image signal supplied via a signal line SL.When each pixel 2 is then selected by a scanning line DS, a lightemitting element in the pixel 2 is driven in accordance with the sampledimage signal. When each pixel 2 is scanned by scanning lines AZ1 andAZ2, a predetermined correction operation is performed.

FIG. 2 is a circuit diagram illustrating a specific example of aconfiguration of the display apparatus of the first comparative exampleshown in FIG. 1. In FIG. 2, for ease of understanding, only one pixelcircuit 2 is shown in an enlarged manner. Each pixel circuit 2 includesfive thin film transistors Tr1 to Tr4 and Trd, one capacitor (storagecapacitor) Cs, and one light emitting element EL. The transistors Tr1 toTr3 and Trd are N-channel type polysilicon TFTs. On the other hand, thetransistor Tr4 is a P-channel type polysilicon TFT. The capacitor Csfunctions as a storage capacitor in the pixel circuit 2. The lightemitting element EL is, for example, an organic electroluminescentelement of a diode type having an anode and a cathode. Note that thelight emitting element is not limited to the organic electroluminescentelement of the diode type, but the light emitting element may be of anytype as long as it emits light according to a driving current passingthrough it.

The driving transistor Trd is a key device in the pixel circuit 2. Thegate G of the driving transistor Trd is connected to one end of thestorage capacitor Cs, and the source S thereof is connected to the otherend of the storage capacitor Cs. Furthermore, the gate G of the drivingtransistor Trd is connected to a reference potential Vss1 via aswitching transistor Tr2. The drain of the driving transistor Trd isconnected to a power supply Vcc via a switching transistor Tr4. The gateof the switching transistor Tr2 is connected to one of the scanninglines AZ1. The gate of the switching transistor Tr4 is connected to oneof the scanning lines DS. The anode of the light emitting element EL isconnected to the source S of the driving transistor Trd, and the cathodeof the light emitting element EL is grounded to a ground potentialVcath. The switching transistor Tr3 is connected between the source S ofthe driving transistor Trd and a reference potential Vss2. The gate ofthe transistor Tr3 is connected to one of the scanning lines AZ2. Thesampling transistor Tr1 is connected between the signal line SL and thegate G of the driving transistor Trd. The gate of the samplingtransistor Tr1 is connected to one of the scanning lines WS.

In the pixel circuit 2 configured in the above-described manner, thesampling transistor Tr1 turns on in response to the control signal WSsupplied via the scanning line WS during a sampling period, whereby theimage signal Vsig supplied via the signal line SL is sampled in thestorage capacitor Cs. The image signal Vsig sampled in the storagecapacitor Cs is applied as an input voltage Vgs between the gate G andthe source S of the driving transistor. The driving transistor Trdsupplies an output current Ids corresponding to the input voltage Vgs tothe light emitting element EL over the light emission period. Note thatthe output current (drain current) Ids depends on the carrier mobility μof the channel region of the driving transistor Trd and also on thethreshold voltage Vth of the driving transistor Trd. The light emittingelement EL is driven by the output current Ids supplied from the drivingtransistor Trd and emits light with intensity corresponding to thesupplied image signal Vsig.

The pixel circuit 2 has a correction mechanism implemented by theswitching transistors Tr2 to Tr4 to eliminate the effects of thedependence of the output current Ids on the carrier mobility μ byadjusting the input voltage Vgs stored in the storage capacitor Csbefore the light emission period. More specifically, the correctionmechanism (Tr2 to Tr4) operates during a part of the sampling period inaccordance with the control signals WS and DS supplied via the scanninglines WS and DS such that when the image signal Vsig is being sampled,the output current Ids of the driving transistor Trd is negatively fedback to the storage capacitor Cs so as to adjust the input voltage Vgs.Furthermore, to eliminate the effects of the dependence of the outputcurrent Ids on the threshold voltage Vth, before the start of thesampling period, the correction mechanism (Tr2 to Tr4) detects thethreshold voltage Vth of the driving transistor Trd and adds thedetected threshold voltage Vth to the input voltage Vgs.

In the present comparative example, the driving transistor Trd is of theN-channel type, and is connected such that the drain thereof isconnected to the power supply Vcc and the source S is connected to thelight emitting element EL. In this configuration, during a partialperiod at the end of the sampling period, the correction mechanismoperates as follows. The output current Ids of the driving transistorTrd is negatively fed back to the storage capacitor Cs such that theoutput current Ids extracted from the source S of the driving transistorTrd is fed into the capacitance of the light emitting element EL beforethe light emission period. More specifically, the light emitting elementEL is of the diode type having an anode and a cathode, wherein the anodeis connected to the source S of the driving transistor Trd, and thecathode is grounded. In this configuration, the correction mechanism(Tr2 to Tr4) operates such that the light emitting element EL isreversely biased between its anode and cathode in advance so that thediode-type light emitting element EL functions as a capacitor when theoutput current Ids extracted from the source S of the driving transistorTrd is fed into the light emitting element EL. This correction mechanismis configured such that the length of the period in which the outputcurrent Ids is extracted from the driving transistor Trd can be adjustedwithin the sampling period to optimize the amount of feedback of theoutput current Ids to the storage capacitor Cs.

FIG. 3 is a timing chart associated with the operation of the displayapparatus of the first comparative example shown in FIG. 2. Referring toFIG. 3, the operation of display apparatus shown in FIG. 2 is describedbelow. FIG. 3 shows waveforms, along time T, of control signals appliedto the scanning lines WS, AZ1, AZ2, and DS. For simplicity, the controlsignals are denoted by the same symbols as those denoting the scanninglines. Because the transistors Tr1, Tr2, and Tr3 are of the N-channeltype, each of the transistors Tr1, Tr2, and Tr3 turns on when thescanning line WS, AZ1, or AZ2 is at a high level and turns off when thescanning line WS, AZ1, or AZ2 is at a low level. On the other hand, thetransistor Tr4 is of the P-channel type, and thus the transistor Tr4turns off when the scanning line DS is at a high level, and thetransistor Tr4 turns on when the scanning line DS is at a low level. Inthe timing chart, in addition to the waveforms of the control signalsWS, AZ1, AZ2, and DS, changes in voltages of the gate G and the source Sof the driving transistor Trd are also shown.

In the timing chart shown in FIG. 3, a period from time T1 to time T8corresponds to one field (1 f) in which the rows of the pixel array aresequentially scanned once. Note that in the timing chart, the waveformsof the control signals WS, AZ1, AZ2, and DS applied to pixels are shownonly for one row.

At time T0 before the start of the field, the control lines WS, AZ1,AZ2, and DS are at the low level. Therefore, the N-channel transistorsTr1, Tr2, and Tr3 are in the off-state, while the P-channel transistorTr4 is in the on-state. Thus, the driving transistor Trd is electricallyconnected to the power supply Vcc via the transistor Tr4 being in theon-state, and the output current Ids depending on the input voltage Vgsis supplied to the light emitting element EL. Thus, at time T0, thelight emitting element EL is emitting light. In this state, the inputvoltage Vgs applied to the driving transistor Trd is given by thedifference between the gate potential (G) and the source potential (S).

At time T1 at which the field starts, the control signal DS rises upfrom the low level to the high level. As a result, the transistor Tr4turns off, and the driving transistor Trd is electrically disconnectedfrom the power supply Vcc. Thus, the emission of light is stopped, andthe operation enters a non-light emission period. Thus, at time T1, alltransistors Tr1 to Tr4 turn off.

Thereafter, at time T2, the control signals AZ1 and AZ2 rise up to thehigh level, and thus the switching transistor Tr2 and Tr3 turn on. As aresult, the gate G of the driving transistor Trd is connected to thereference potential Vss1, and the source S is connected to the referencepotential Vss2. In this state, parameters are set Vss1−Vss2>Vth andVss1−Vss2=Vgs>Vth are satisfied so as to make it possible to properlyperform the Vth correction process in a following period starting fromtime T3. In other words, the period from T2 to T3 functions as aresetting period in which the driving transistor Trd is reset.Furthermore, when the threshold voltage of the light emitting element ELis denoted by VthEL, parameters are set such that VthEL>Vss2 issatisfied, and thus the light emitting element EL is negatively biased.This negative biasing is necessary to make proper corrections in termsof Vth and the carrier mobility, in a later processing step.

At about time. T3, the control signal AZ2 is switched to the low level,and immediately thereafter, the control signal DS is also switched tothe low level. As a result, the transistor Tr3 turns off and thetransistor Tr4 turns on. Thus, the drain current Ids flows into thestorage capacitor Cs, and the Vth correction operation starts. In thiscorrection operation, the gate G of the driving transistor Trd ismaintained at Vss1, and the current Ids flows until the drivingtransistor Trd turns off. When the driving transistor Trd turns off, thesource potential (S) of the driving transistor Trd is equal to Vss1−Vth.At time T4 after the drain current is cut off, the control signal DS isreturned to the high level to turn off the switching transistor Tr4. Thecontrol signal AZ1 is then also returned to the low level to turn offthe switching transistor Tr2. As a result, Vth is stored and retained inthe storage capacitor Cs. As described above, the threshold voltage Vthof the driving transistor Trd is detected in the period from T3 to T4.Thus, hereinafter, the period T3 to T4 will be referred to as a Vthcorrection period.

At time T5 after completion of the Vth correction, the control signal WSis switched to the high level to turn on the sampling transistor Tr1thereby writing the image signal Vsig to the storage capacitor Cs. Thecapacitance of the storage capacitor Cs is much smaller than thecapacitance of the equivalent capacitance Coled of the light emittingelement EL. As a result, most part of the image signal Vsig is writtenin the storage capacitor Cs. More specifically, the voltage of the imagesignal Vsig with respect to Vss1, that is, Vsig−Vss1 is written in thestorage capacitor Cs. Thus, the voltage Vgs between the gate G and thesource S of the driving transistor Trd is given by Vth detected in theprevious step plus the sampled voltage Vsig−Vss1, that is,Vsig−Vss1+Vth. In the following discussion, for simplicity, let usassume that Vss1=0 V. In this case, as shown in the timing chart of FIG.3, the gate-source voltage Vgs is given by Vsig+Vth. The sampling of theimage signal Vsig is continued until the control signal WS is returnedto the low level at time T7. Thus, the period from T5 to T7 functions asthe sampling period.

At time T6 before the end T7 of the sampling period, the control signalDS falls down to the low level. As a result, the switching transistorTr4 turns on. Thus, the driving transistor Trd is electrically connectedto the power supply Vcc, and the pixel circuit proceeds from thenon-light emission period to a light emission period. In the period fromT6 to T7 in which the sampling transistor Tr1 is still in the on-stateand the switching transistor Tr4 is turned on, the correction is made asto the carrier mobility of the driving transistor Trd. That is, in thepresent comparative example, the carrier mobility correction isperformed in the period from T6 to T7 in the tail of the samplingperiod. At the beginning of the period in which the carrier mobilitycorrection is performed, the light emitting element EL is actually inthe reverse-biased state, and thus no light is emitted. In this mobilitycorrection period from T6 to T7, the drain current Ids flows through thedriving transistor Trd in the state in which the gate G of the drivingtransistor Trd is maintained at the voltage of the image signal Vsig. Inthis state, Vss1−Vth<VthEL is satisfied, that is, the light emittingelement EL is reversely biased, and thus the light emitting element ELfunctions not as a diode but as a capacitor. Thus, the current Idspassing through the driving transistor Trd is written into the totalcapacitor of the storage capacitor Cs and the equivalent capacitorColed. As a result, the source potential (S) of the driving transistorTrd rises up. In the timing chart shown in FIG. 3, the increase in thesource potential is denoted by ΔV. The gate-source voltage Vgs stored inthe storage capacitor Cs is reduced by the increase in the sourcepotential ΔV, and thus negative feedback occurs. As a result of thenegative feedback of the output current Ids of the driving transistorTrd to the input voltage Vgs of the driving transistor Trd, thecorrection associated with the mobility μ is performed. The amount ofthe negative feedback ΔV can be optimized by adjusting the length t ofthe mobility correction period from T6 to T7.

At time T7, the control signal WS falls down to the low level, and thusthe sampling transistor Tr1 turns off. As a result the gate G of thedriving transistor Trd is electrically disconnected from the signal lineSL. Thus, the image signal Vsig is no longer applied to the gate G ofthe driving transistor Trd, and the gate potential (G) of the drivingtransistor Trd rises up with the source potential (S). In thistransition process, the gate-source voltage Vgs is maintained at thevoltage stored in the storage capacitor Cs, i.e., (Vsig−ΔV+Vth). As aresult of the increase in the source potential (S), the bias of thelight emitting element EL is switched from the reverse bias to theforward bias, and the output current Ids flows through the lightemitting element EL. Thus, the light emitting element EL starts emittinglight. In this state, the dependence of the drain current Ids on thegate voltage Vgs can be obtained by substituting Vsig−ΔV+Vth into Vgs inthe transistor characteristic equation described above, that is, theoutput current Id is given byIds=kμ(Vgs−Vth)² =kμ(Vsig−ΔV)²wherek=(1/2)(W/L)CoxAs can be seen from the above equation, the resultant output current Idsno longer includes a term of Vth, that is, the output current Idssupplied to the light emitting element EL does not depend on thethreshold voltage Vth of the driving transistor Trd. Thus, the draincurrent Ids is basically determined by the signal voltage Vsig of theimage signal. In other words, the light emitting element EL emits lightwith intensity corresponding to the image signal Vsig corrected by thefeedback ΔV. The amount of correction value ΔV is determined so as toeliminate the effect of the mobility g in the coefficient of thecharacteristic equation. As a result, the drain current Ids depends onlyon the image signal Vsig.

At time T8, the control signal DS rises up to the high level, and theswitching transistor Tr4 turns off. As a result, light emission isstopped, and the present field ends. Thereafter, a next field issubjected to the similar operation including the Vth correction process,the mobility correction process, and the light emission process.

FIG. 4 illustrates an example of the current-voltage characteristic ofthe light emitting element EL. In this figure, a vertical axisrepresents the current Ioled and a horizontal axis represents thevoltage Voled. In the case of the light emitting element included in thepixel circuit, Ioled is given by the drain current Ids supplied from thedriving transistor Trd, and Voled is given by the source potential ofthe driving transistor (i.e., the anode potential of the light emittingelement). As can be seen from the graph shown in FIG. 4, when Voled isnegative and thus when the light emitting element is negatively biased,the light emitting element turns off as with usual diodes, and nocurrent flows other than a leakage current. The leakage current variesfrom one light emitting element to another, i.e., some light emittingelement has a large current as represented by a dotted line in FIG. 4,while some light emitting element has a small leakage current asrepresented by a solid line.

In the display apparatus of the first comparative example, the thresholdcorrection process and the mobility correction process are performed inthe state in which the light emitting element is reversely biased. Whenthe light emitting element is in the reverse-biased state, a leakagecurrent flows in the reverse direction as described above. The leakagecurrent can cause the source potential of the driving transistor tochange in the threshold voltage correction process or the mobilitycorrection process, and thus an error can occur in the corrections. Asshown in FIG. 4, the bias characteristic of the light emitting elementEL varies from one element to another. This means that there can be alight emitting element having a relatively large leakage currentcompared with other light emitting elements. In a pixel including such alight emitting element having a large leakage current, the large leakagecurrent flowing through the pixel circuit causes a large current to flowinto the source of the driving transistor Trd in the mobility correctionprocess or the threshold voltage correction process, and thus areduction in Vgs stored in the storage capacitor Cs occurs. This leadsto a reduction in the intensity of light emitted by this pixel. If thislight emitting element EL is continuously operated, the leakagecharacteristic can change with time, which causes a change in intensitywith time.

In the display apparatus of the first comparative example, the sourcepotential of the driving transistor Trd increases during the mobilitycorrection process. To achieve the accurate correction, the sourcevoltage of the driving transistor needs to have a value which does notturn on the light emitting element after completion of the mobilitycorrection process. To this end, the cathode potential needs to be setto be relatively high with respect to the signal potential. However, thesetting of the cathode potential to a relatively high value with respectto the signal potential needs a large power supply voltage applied tothe panel, which leads to an increase in power consumption. The sourcepotential of the driving transistor can be maintained within the rangewhich does not cause the light emitting element to turn on at the pointof time at which the mobility correction process is ended, if the signalpotential is set to be relatively low with respect to the cathodevoltage. However, in this case, a negative input voltage is applied tothe gate of the driving transistor, which leads to an increase in costfor the driver which outputs the image signal. As can be seen from theabove discussion, to achieve a panel with high image quality withoutincreasing power consumption or cost, it is desirable to avoid theeffects of the leakage currents of the light emitting elements on thelight emission intensity.

FIG. 5 is a circuit diagram of a display apparatus according to a firstembodiment of the present invention. The circuit of the displayapparatus according to the present embodiment is configured to solve theproblems in the first comparative example described above. In FIG. 5,for ease of understanding, similar parts to those in FIG. 2 are denotedby similar reference symbols. This circuit is different from that of thefirst comparative example in that a switching transistor Tr6 is disposedbetween the source S of the driving transistor Trd and the anode of thelight emitting element EL. To control the gate of this switchingtransistor Tr6, additional scanning lines DS2 extending parallel to thescanning lines WS are disposed in the pixel array 1. Correspondingly, tosequentially supply a control signal to the scanning lines DS2, thedriving unit disposed in the peripheral area additionally includessecond drive scanner 8. The switching transistor Tr6 is maintained inthe off-state during the threshold voltage correction period and themobility correction period thereby eliminating the effects of theleakage current of the light emitting element EL and thus avoiding thedegradation in uniformity of intensity across the screen. Furthermore,because the driving transistor Trd is electrically disconnected from thelight emitting element EL, it becomes possible to set the cathodepotential of the light emitting element EL and the signal voltageapplied to the driving transistor Trd independently of each other, whichallows a reduction in power consumption of the panel and a reduction incost.

As shown in FIG. 5, in the first embodiment of the present invention,the display apparatus basically includes a pixel array 1 and a drivingunit adapted to drive the pixel array 1. The pixel array 1 includesscanning lines WS extending in the row direction, signal lines SLextending in the column direction, and pixels 2 disposed in an arrayform at respective intersections of the scanning lines WS and the signallines SL. Each pixel 2 includes, at least, a sampling transistor Tr1, adriving transistor Trd, a light emitting element EL, and a storagecapacitor Cs. The control terminal (the gate) of the sampling transistorTr1 is connected to one of the scanning lines WS. The current terminals(the source and the drain) of the sampling transistor Tr1 arerespectively connected to one of the signal lines SL and the controlterminal (the gate G) of the driving transistor Trd. One of the currentterminals (the source and the drain) of the driving transistor Trd isconnected to the light emitting element EL, and the other one isconnected to a power supply line Vcc. More specifically, in the presentembodiment, the driving transistor Trd is of the N-channel type, andthus the source S is connected to the anode of the light emittingelement EL. The storage capacitor Cs is connected between the controlterminal (the gate G) and the output current terminal (the source S) ofthe driving transistor Trd. An auxiliary capacitor Csub is connectedbetween the source S of the driving transistor Trd and a ground line.

The driving unit includes a write scanner 4 adapted to output a controlsignal over the scanning line WS thereby to turn on the samplingtransistor Tr1. The driving unit also includes a horizontal selector 3adapted to output an image signal over the signal line SL such that theimage signal is written in the storage capacitor Cs via the turned-onsampling transistor Tr1. Thus, the driving transistor Trd operates suchthat the driving current Ids corresponding to the image signal writtenas the signal voltage is supplied to the light emitting element ELduring the light emission period, while no driving current is suppliedto the light emitting element EL during the non-light emission period.

The present embodiment is characterized that each pixel circuit 2 has acorrection mechanism and a switching transistor Tr6. The correctionmechanism is implemented by correction transistors Tr2, Tr3, and Tr4 andother elements, and operates during the non-light emission period suchthat a correction voltage is written in the storage capacitor Cs therebyto cancel out the variation in characteristics of the drivingtransistors Trd. The switching transistor Tr6 is located between thesource of the driving transistor Trd and the anode of the light emittingelement EL, and the switching transistor Tr6 is turned off during thenon-light emission period whereby the light emitting element EL iselectrically disconnected from the source S of the driving transistorTrd so that no leakage current flows into the light emitting element ELduring the period in which the correction mechanism thereby preventingan error due to the leakage current.

More specifically, the correction mechanism (Tr2, Tr3, and Tr4) operatessuch that the correction voltage corresponding to the threshold voltageVth is added to the voltage stored in the storage capacitor Cs therebyeliminating the effects of the variation in the threshold voltage Vth ofthe driving transistor Trd. Furthermore, the correction mechanismsubtracts a correction voltage corresponding to the mobility μ from theimage signal written as the signal voltage in the storage capacitor Csthereby eliminating the effects of the variation in the mobility μ ofthe driving transistor Trd.

As can be seen from the above discussion, the source S of the drivingtransistor Trd is disconnected from the light emitting element EL duringthe mobility correction period. Therefore, even if the source potentialof the driving transistor Trd increases to a value that otherwise wouldturn on the light emitting element EL in the mobility correction period,no problem occurs in the operation. Thus, in the present embodiment,unlike the first comparative example described above, it is allowed toset the cathode voltage of the light emitting element EL and the signalvoltage applied to the gate G of the driving transistor Trdindependently of each other. Therefore, the image signal is allowed tobe set in a positive range, which allows a reduction in cost of thedriver of the horizontal selector 3. Furthermore, the cathode voltage isallowed to be set to the ground voltage, which allows the powerconsumption of the panel to be minimized.

With reference to FIG. 6, the operation of the display apparatusaccording to the first embodiment shown in FIG. 5 is described infurther detail below. In FIG. 6, for ease of understanding, similarreference symbols to those in the timing chart shown in FIG. 3associated with the first comparative example are used. At time T0before the start of each field, the control lines WS, AZ1, AZ2, and DSare all at the low level. Therefore, the N-channel transistors Tr1, Tr2,and Tr3 are in the off-state, while the P-channel transistor Tr4 is inthe on-state. Thus, the driving transistor Trd is electrically connectedto the power supply Vcc via the transistor Tr4 being in the on-state,and the output current Ids depending on the input voltage Vgs issupplied to the light emitting element EL. Thus, at time T0, the lightemitting element EL is emitting light. In this state, the input voltageVgs applied to the driving transistor Trd is given by the differencebetween the gate potential (G) and the source potential (S).

At time T1 at which the field starts, the control signal DS rises upfrom the low level to the high level. As a result, the transistor Tr4turns off, and the driving transistor Trd is electrically disconnectedfrom the power supply Vcc. Thus, the emission of light is stopped, andthe operation enters a non-light emission period. Thus, at time T1, alltransistors Tr1 to Tr4 turn off.

Thereafter, at time T1 a, the control signal DS2 is switched to the lowlevel to turn off the switching transistor Tr6. As a result, the lightemitting element EL is electrically disconnected from the drivingtransistor Trd during the non-light emission period. Although in thepresent embodiment, the switching transistor Tr4 is turned off at timeT1 and then the switching transistor Tr6 is turned off, the switchingtransistor Tr6 may be turned off first and then the switching transistorTr4 may be turned off. Note that the light emitting element EL should bedisconnected before the correction switching transistors Tr2 and Tr3 areturned on.

Thereafter, at time T2, the control signals AZ1 and AZ2 rise up to thehigh level, and thus the switching transistor Tr2 and Tr3 turn on. As aresult, the gate G of the driving transistor Trd is connected to thereference potential Vss1, and the source S is connected to the referencepotential Vss2. In this state, parameters are set Vss1−Vss2>Vth andVss1−Vss2=Vgs>Vth are satisfied so as to make it possible to properlyperform the Vth correction process in a following period starting fromtime T3. At this point of time, the light emitting element EL hasalready been disconnected from the source S of the driving transistorTrd. Therefore, the light emitting element EL does not need to bereversely biased.

At time T3, the control signal AZ2 is switched to the low level, andimmediately thereafter, the control signal DS is also switched to thelow level. As a result, the transistor Tr3 turns off and the transistorTr4 turns on. As a result, the drain current Ids flows into the storagecapacitor Cs, and the Vth correction operation starts. In this state,the gate G of the driving transistor Trd is maintained at Vss1, and thecurrent Ids flows until the driving transistor Trd turns off. When thedriving transistor Trd turns off, the source potential (S) of thedriving transistor Trd is equal to Vss1−Vth. At time T4 after the draincurrent is cut off, the control signal DS is returned to the high levelto turn off the switching transistor Tr4. The control signal AZ1 is thenalso returned to the low level to turn off the switching transistor Tr2.As a result, Vth is stored and retained in the storage capacitor Cs. Asdescribed above, the threshold voltage Vth of the driving transistor Trdis detected in the period from T3 to T4. Thus, hereinafter, the periodT3 to T4 will be referred to as a Vth correction period.

At time T5 after completion of the Vth correction, the control signal WSis switched to the high level to turn on the sampling transistor Tr1thereby writing the image signal Vsig to the storage capacitor Cs. Thecapacitance of the storage capacitor Cs is much smaller than thecapacitance of the auxiliary capacitance Csub. As a result, most of theimage signal Vsig is written in the storage capacitor Cs. Morespecifically, the voltage of the image signal Vsig with respect to Vss1,that is, Vsig−Vss1 is written in the storage capacitor Cs. Therefore,the voltage Vgs between the gate G and the source S of the drivingtransistor Trd is given by Vth detected in the previous step plus thesampled voltage Vsig−Vss1, that is, Vsig−Vss1+Vth. In the followingdiscussion, for simplicity, let us assume that Vss1=0 V. In this case,as shown in the timing chart of FIG. 6, the gate-source voltage Vgs isgiven by Vsig+Vth. The sampling of the image signal Vsig is continueduntil the control signal WS is returned to the low level at time T7.Thus, the period from T5 to T7 functions as the sampling period.

At time T6 before the end T7 of the sampling period, the control signalDS falls down to the low level. As a result, the switching transistorTr4 turns on. Thus, the driving transistor Trd is electrically connectedto the power supply Vcc. In the period from T6 to T7 in which thesampling transistor Tr1 is still in the on-state and the switchingtransistor Tr4 is turned on, the correction is made as to the carriermobility of the driving transistor Trd. Note that in this period, thelight emitting element EL is still disconnected from the source S of thedriving transistor Trd. In this mobility correction period from T6 toT7, the drain current Ids flows through the driving transistor Trd inthe state in which the gate G of the driving transistor Trd ismaintained at the voltage of the image signal Vsig. Thus, the currentIds passing through the driving transistor Trd is written into the totalcapacitor of the storage capacitor Cs and the auxiliary capacitor, i.e.,C=Cs+Csub. As a result, the source potential (S) of the drivingtransistor Trd rises up. The gate-source voltage Vgs stored in thestorage capacitor Cs is reduced by the increase in the source potentialΔV, and thus negative feedback occurs. As a result of the negativefeedback of the output current Ids of the driving transistor Trd to theinput voltage Vgs of the driving transistor Trd, the effects of thevariation in the mobility μ are eliminated.

At time T7, the control signal WS falls down to the low level, and thusthe sampling transistor Tr1 turns off. As a result, the gate G of thedriving transistor Trd is disconnected from the signal line SL. Thus,the image signal Vsig is no longer applied to the gate G of the drivingtransistor Trd, and the gate potential (G) of the driving transistor Trdrises up with the source potential (S). More specifically, in thepresent embodiment, because the source S of the driving transistor Trdis disconnected from the light emitting element EL, the source potentialincreases until it becomes substantially equal to the power supplyvoltage Vcc. Thus, in response to the rising of the source potential,the gate potential of the driving transistor Trd also rises. In thistransition process, the gate-source voltage Vgs is maintained at thevoltage stored in the storage capacitor Cs, i.e., (Vsig−ΔV+Vth).

Thereafter, at time T7 a, the control signal DS2 rises up to the highlevel. As a result, the switching transistor Tr6 turns on, and the lightemitting element EL is electrically connected to the driving transistorTrd. As a result, the driving current Ids supplied from the drivingtransistor Trd flows into the light emitting element EL, and light isemitted by the light emitting element EL. In this process, the sourcepotential of the driving transistor Trd (and thus also the anodepotential of the light emitting element EL) falls down to a leveldetermined by operating points of the driving transistor Trd and thelight emitting element EL and remains at this stable level over thefollowing light emission period.

Finally, at time T8, the control signal DS rises up to the high level,and the switching transistor Tr4 turns off. As a result, light emissionis stopped, and the present field ends. Thereafter, a next field issubjected to the similar operation including the Vth correction process,the mobility correction process, and the light emission process.

FIG. 7 is a circuit diagram of a second comparative example of a displayapparatus. In this second comparative example, each pixel is basicallycomposed of four transistors, one storage capacitor, and one lightemitting element. Note that in this second comparative example, incontrast to the first comparative example shown in FIG. 2 in which fivetransistors are used, only four transistors are used. The drivingtransistor Trd is a key device in the pixel circuit 2. The gate G of thedriving transistor Trd is connected to one end of the storage capacitorCs, and the source S thereof is connected to the other end of thestorage capacitor Cs. The drain of the driving transistor Trd isconnected to a power supply Vcc via a switching transistor Tr4. The gateof the switching transistor Tr4 is connected to one of the scanninglines DS. The anode of the light emitting element EL is connected to thesource S of the driving transistor Trd, and the cathode of the lightemitting element EL is grounded to a ground potential Vcath. A secondswitching transistor Tr3 is connected between the source S of thedriving transistor Trd and a reference potential Vss. The gate of thetransistor Tr3 is connected to one of the scanning lines AZ. A samplingtransistor Tr1 is connected between the gate G of the driving transistorTrd and the signal line SL. The gate of the sampling transistor Tr1 isconnected to one of the scanning lines WS.

In this configuration, in a horizontal scanning period (1H) assigned toa scanning line WS, the sampling transistor Tr1 turns on in response tothe control signal WS supplied via the scanning line WS whereby theimage signal Vsig supplied via the signal line SL is sampled in thestorage capacitor Cs. The image signal Vsig sampled in the storagecapacitor Cs is applied as an input voltage Vgs to the gate G of thedriving transistor Trd. The driving transistor Trd supplies an outputcurrent Ids corresponding to the input voltage Vgs to the light emittingelement EL over the light emission period. Note that the output currentIds depends on the threshold voltage Vth of the driving transistor Trd.The light emitting element EL is driven by the output current Idssupplied from the driving transistor Trd and emits light with intensitycorresponding to the supplied image signal Vsig.

The pixel circuit 2 has a correction mechanism implemented by the firstswitching transistor Tr3 and the second switching transistor Tr4. Thecorrection mechanism operates in a partial period in the horizontalscanning period (1H) to eliminate the effects of the dependence of theoutput current Ids on the threshold voltage Vth. More specifically, thethreshold voltage Vth of the driving transistor Trd is detected andwritten in the storage capacitor Cs. The correction mechanism operatesin the state in which the sampling transistor Tr1 turns on in thehorizontal scanning period (1H) and one end of the storage capacitor Csis maintained at the fixed voltage Vss0 via the signal line SL. In thisstate, the storage capacitor Cs is charged by a voltage applied to theother end of the storage capacitor Cs until the voltage on the other endof the storage capacitor Cs with respect to the fixed voltage Vss0becomes equal to the threshold voltage Vth. After the threshold voltageVth of the driving transistor Trd is detected and written in the storagecapacitor Cs in the first half of the horizontal scanning period (1H),then in the second half of the horizontal scanning period, the samplingtransistor Tr1 samples the image signal Vsig supplied via the signalline SL and writes the sampled signal voltage in the storage capacitorCs. As a result, the input voltage Vgs equal to the sum of the sampledimage signal Vsig and the previously written threshold voltage Vth isstored in the storage capacitor Cs, and this total voltage storedtherein is applied between the gate G and the source S of the drivingtransistor Trd whereby the dependence of the output current Ids on thethreshold voltage Vth is eliminated. The correction mechanism includes afirst switching transistor Tr3 that turns on before the start of eachhorizontal scanning period (1H) thereby to reset the voltage across thestorage capacitor Cs to a value greater than the threshold voltage Vth.The correction mechanism also includes a second switching transistor Tr4that turns on in each horizontal scanning period (1H) thereby to chargethe storage capacitor Cs until the voltage across the storage capacitorCs becomes equal to the threshold voltage Vth. In the signal supplyperiod, in the horizontal scanning period (1H), in which the signal lineSL is at the potential of the image signal Vsig, the sampling transistorTr1 samples the image signal Vsig supplied via the signal line SL at thestorage capacitor Cs, while in the fixed signal period, in thehorizontal scanning period (1H), in which the signal line SL is at thefixed potential Vss0, the threshold voltage Vth of the drivingtransistor Trd is detected and written in the storage capacitor Cs.

In the present comparative example, the output current Ids of thedriving transistor Trd depends not only on the threshold voltage Vth butalso on the carrier mobility μ of the channel region of the drivingtransistor. To handle the above-described dependence, the correctionmechanism operates in the partial period of the horizontal scanningperiod (1H) such that the dependence of the output current Ids on thecarrier mobility μ is eliminated by extracting the output current Ids ofthe driving transistor Trd when the image signal Vsig is being sampled,and negatively feeding the output current Ids back to the storagecapacitor Cs thereby correcting the input voltage Vgs.

FIG. 8 is a timing chart provided for an explanation of the operation ofthe second comparative example of the circuit shown in FIG. 7. FIG. 8illustrates waveforms, along time T, of control signals applied to thescanning lines WS, AZ, and DS. For simplicity, the control signals aredenoted by the same symbols as those denoting the scanning lines. InFIG. 8, the waveform of the image signal Vsig applied to the signal lineis also shown along the time axis T. As shown in FIG. 8, the imagesignal Vsig has a fixed potential value Vss0 in the first half of eachhorizontal scanning period H and has a signal potential in the secondhalf. Because the transistors Tr1 and Tr3 are of the N-channel type,these transistors turn on when the scanning line WS or AZ is at the highlevel, while they turn off when the scanning line WS or AZ is at the lowlevel. On the other hand, the P-channel type transistor Tr4 turns offwhen the scanning line DS is at the high level and turns on when thescanning line DS is at the low level. In the timing chart of FIG. 8, inaddition to the waveforms of the control signals WS, AZ, and DS and thewaveform of the image signal Vsig, changes in voltages of the gate G andthe source S of the driving transistor Trd are also shown.

In the timing chart shown in FIG. 8, a period from time T1 to time T8corresponds to one field (1 f) in which the rows of the pixel array aresequentially scanned once. Note that in the timing chart, the waveformsof the control signals WS, AZ, and DS applied to pixels are shown onlyfor one row.

At time T0 before the start of the field, the control lines WS, AZ, andDS are all at the low level. Therefore, the N-channel transistors Tr1and Tr3 are in the off-state, and the P-channel transistor Tr4 is in theon-state. Thus, the driving transistor Trd is electrically connected tothe power supply Vcc via the transistor Tr4 being in the on-state, andthe output current Ids depending on the input voltage Vgs is supplied tothe light emitting element EL. Thus, at time T0, the light emittingelement EL is emitting light. In this state, the input voltage Vgsapplied to the driving transistor Trd is given by the difference betweenthe gate potential (G) and the source potential (S).

At time T1 at which the field starts, the control signal DS rises upfrom the low level to the high level. As a result, the transistor Tr4turns off, and the driving transistor Trd is electrically disconnectedfrom the power supply Vcc. Thus, the emission of light is stopped, andthe operation enters a non-light emission period. At time T1, alltransistors Tr1, Tr3, and Tr4 turn off.

Thereafter, at time T2, the control signal AZ rises up from the lowlevel to the high level. As a result, the switching transistor Tr3 turnson. Thus, the reference voltage Vss is applied to the other end of thestorage capacitor Cs and to the source S of the driving transistor Trd.In this process, because the gate of the driving transistor Trd has ahigh impedance, the gate potential (G) also falls down with falling-downsource potential (S).

Thereafter, the control signal AZ returns to the low level, and theswitching transistor Tr3 turns off. At time Ta, the control signal WSrises up to the high level, and thus the sampling transistor Tr1 turnson. At this point of time, the signal line is at the fixed potentialVss0. Note that Vss0 and Vss are set such that Vss0−Vss>Vth issatisfied. Also note that Vss0−Vss is applied as the input voltage Vgsto the driving transistor Trd. Parameters are set such that Vgs>Vth issatisfied so as to make it possible to properly perform the Vthcorrection process in a following period. In other words, at time Ta,the voltage across the storage capacitor Cs is reset to a value greaterthan Vgs before the Vth correction process starts. Furthermore, when thethreshold voltage of the light emitting element EL is denoted by VthEL,Vss is set such that VthEL>Vss is satisfied, i.e., the light emittingelement EL is reversely biased. The above settings are necessary toproperly perform the following Vth correction process.

Thereafter, at time T3, the control signal DS is switched to the lowlevel. As a result, the switching transistor Tr4 turns on and the Vthcorrection operation starts. To achieve high accuracy in the Vthcorrection process, the potential of the signal line is still maintainedat the fixed potential Vss0. The turning-on of the switching transistorTr4 causes the driving transistor Trd to be electrically connected tothe power supply Vcc, and thus the output current Ids flows. As aresult, the storage capacitor Cs is charged and the source potential (S)connected to one end of the storage capacitor Cs goes up. On the otherhand, the potential of the other end of the storage capacitor Cs (i.e.,the gate potential (G)) is fixed at Vss0. Thus, as the storage capacitorCs is charged, the source potential (S) rises up, and the drivingtransistor Trd turns off when the input voltage Vgs reaches Vth. If thedriving transistor Trd turns off, the source potential (S) thereofbecomes equal to Vss0−Vth as shown in the timing chart.

Thereafter, at time T4, the control signal DS is returned to the highlevel to turn off the switching transistor Tr4. Thus, the Vth correctionprocess is completed. As a result of the correction process, a voltagecorresponding to the threshold voltage Vth is stored in the storagecapacitor Cs.

After the completion of the Vth correction process in the period fromtime T3 to time T4, when the first half of the horizontal scanningperiod (1H) has elapsed, the potential of the signal line is switchedfrom Vss0 to Vsig. As a result, the image signal Vsig is written in thestorage capacitor Cs. The capacitance of the storage capacitor Cs ismuch smaller than the capacitance of the equivalent capacitance Coled ofthe light emitting element EL. Therefore, most part of the image signalVsig is written in the storage capacitor Cs. Thus, the voltage Vgsbetween the gate G and the source S of the driving transistor Trd isgiven by Vth detected in the previous step plus the sampled voltageVsig, i.e., Vsig+Vth. Thus, as shown in the timing chart of FIG. 8, thegate-source voltage Vgs is given by Vsig+Vth. The sampling of the imagesignal Vsig is continued until the control signal WS is returned to thelow level at time T7. Thus, the period from T5 to T7 functions as thesampling period.

In the present comparative example, as described above, the Vthcorrection period from T3 to T4 and the sampling period from T5 to T7are included in each horizontal scanning period (1H). Over eachhorizontal scanning period (1H), the sampling control signal WS ismaintained at the high level. In the present comparative example, theVth correction process and the writing of Vsig are performed in thestate in which the sampling transistor Tr1 is in the on-state. Thisallows simplification of the pixel circuit 2.

In the present comparative example, in addition to the Vth correctionprocess, the correction associated with the mobility μ is alsoperformed. However, the configuration is not limited to that describedabove. For example, the correction may be made only with respect to Vthwithout performing the correction with respect to the mobility μ.Furthermore, in the present comparative example of the pixel circuit 2,a mixture of N-channel and P-channel transistors is used for thetransistors. Alternatively, the transistor other than the drivingtransistor Trd may all be of the N-channel type or may all be of theP-channel type.

The correction associated with the mobility μ is performed in the periodfrom T6 to T7 as described in further detail below. At time T6 beforetime T7 at which the sampling period ends, the control signal DS fallsdown to the low level. As a result, the switching transistor Tr4 turnson. Thus, the driving transistor Trd is electrically connected to thepower supply Vcc, and the pixel circuit proceeds from the non-lightemission period to a light emission period. In the period from T6 to T7in which the sampling transistor Tr1 is still in the on-state and theswitching transistor Tr4 is turned on, the correction is made as to thecarrier mobility of the driving transistor Trd. That is, in the presentcomparative example, the carrier mobility correction is performed in theperiod from T6 to T7 in which a tail part of the sampling periodoverlaps a beginning part of the light emission period. In the beginningpart of the light emission period in which the correction associatedwith the mobility is performed, the light emitting element EL isreversely biased and thus no light is emitted. In the mobilitycorrection period from T6 to T7, the drain current Ids flows through thedriving transistor Trd in the state in which the gate G of the drivingtransistor Trd is fixed at the level corresponding to the image signalVsig. In this state, parameters are set such that Vss0−Vth<VthEL issatisfied so that the light emitting element EL is reversely biased, andthus the light emitting element EL functions not as a diode but as acapacitor. Thus, the current Ids passing through the driving transistorTrd is written into the total capacitor of the storage capacitor Cs andthe equivalent capacitor Coled. As a result, the source potential (S) ofthe driving transistor Trd rises up. In the timing chart shown in FIG.8, the increase in the source potential is denoted by ΔV. Thegate-source voltage Vgs stored in the storage capacitor Cs is reduced bythe increase in the source potential ΔV, and thus negative feedbackoccurs. As a result of the negative feedback of the output current Idsof the driving transistor Trd to the input voltage Vgs of the drivingtransistor Trd, the correction associated with the mobility μ isperformed. The amount of the negative feedback ΔV can be optimized byadjusting the length t of the mobility correction period from T6 to T7.

At time T7, the control signal WS falls down to the low level, and thusthe sampling transistor Tr1 turns off. As a result, the gate G of thedriving transistor Trd is disconnected from the signal line SL. Thus,the image signal Vsig is no longer applied to the gate G of the drivingtransistor Trd, and the gate potential (G) of the driving transistor Trdrises up with the source potential (S). In this transition process, thegate-source voltage Vgs is maintained at the voltage stored in thestorage capacitor Cs, i.e., (Vsig−ΔV+Vth). As a result of the increasein the source potential (S), the bias of the light emitting element ELis switched from the reverse bias to the forward bias, and the outputcurrent Ids flows through the light emitting element EL. Thus, the lightemitting element EL starts emitting light. In this state, the dependenceof the drain current Ids on the gate voltage Vgs can be obtained bysubstituting Vsig−ΔV+Vth into Vgs in the transistor characteristicequation described above, that is, the output current Id is given byIds=kμ(Vgs−Vth)² =kμ(Vsig−ΔV)²wherek=(1/2)(W/L)Cox

As can be seen from the above equation, the resultant output current Idsno longer includes a term of Vth, that is, the output current Idssupplied to the light emitting element EL does not depend on thethreshold voltage Vth of the driving transistor Trd. Thus, the draincurrent Ids is basically determined by the signal voltage Vsig of theimage signal. In other words, the light emitting element EL emits lightwith intensity corresponding to the image signal Vsig corrected by thefeedback ΔV. The amount of correction value ΔV is determined so as toeliminate the effect of the mobility μ in the coefficient of thecharacteristic equation. As a result, the drain current Ids depends onlyon the image signal Vsig.

Finally, at time T8, the control signal DS rises up to the high level,and the switching transistor Tr4 turns off. As a result, light emissionis stopped, and the present field ends. Thereafter, a next field issubjected to the similar operation including the Vth correction process,the mobility correction process, and the light emission process.

FIG. 9 is a circuit diagram of a display apparatus according to a secondembodiment of the present invention. This display apparatus isconfigured in a basically similar manner to the second comparativeexample shown in FIG. 7. In FIG. 9, similar parts to those in FIG. 7 aredenoted by similar reference symbols. The display apparatus according tothe second embodiment of the invention is different from the secondcomparative example shown in FIG. 7 in that a switching transistor Tr6is disposed between the source S of the driving transistor Trd and theanode of the light emitting element EL. Furthermore, the equivalentcapacitor Coled of the light emitting element EL is removed, and anauxiliary capacitor Csub is connected between the source S of thedriving transistor Trd and a ground line. To drive the gate of theswitching transistor Tr6, additional scanning lines DS2 are disposed inthe pixel array 1. The scanning lines DS2 are sequentially scanned lineby line by a second drive scanner 8 disposed in a driving unit.

FIG. 10 is a timing chart provided for an explanation of the operationof the circuit according to the second embodiment shown in FIG. 9. InFIG. 6, for ease of understanding, similar reference symbols to those inthe timing chart shown in FIG. 8 are used. As shown in FIG. 10, at timeT1, the switching transistor Tr4 is turned off to switch the pixel 2from the light emission state into the non-light emission state.Thereafter, at time T1 a, the scanning line DS2 is switched to the lowlevel to turn off the switching transistor Tr6. As a result, the lightemitting element EL is electrically disconnected from the drivingtransistor Trd. Thereafter, in a period from T2 to T3, a preliminaryprocess is performed. Furthermore, in a period from T3 to T4, thethreshold voltage correction process is performed. In a period from T5to T7, the image signal is written in the storage capacitor Cs. In thisprocess, the mobility correction is performed in a period from T6 to T7.In these processes, the light emitting element EL is electricallydisconnected from the driving transistor Trd, thereby eliminating theeffects of the leakage current of the light emitting element EL.

If all correction processes are completed at time T7, then at time T7 a,the control signal DS2 is returned to the high level to turn on theswitching transistor Tr6. As a result, the light emitting element EL iselectrically connected to the driving transistor Trd. Thus the outputcurrent Ids is supplied to the light emitting element EL, and the lightemitting element EL starts emitting light. As a result, the potential ofthe source S, which determines the operating point of the drivingtransistor Trd and the light emitting element EL, falls down from thepower supply potential Vcc to a lower level and settles there. Inresponse to the falling of the source potential, the potential of thegate G of the driving transistor Trd also falls down to a lower leveland settles there.

FIG. 11 is a circuit diagram of a third comparative example of a displayapparatus. In this third comparative example, each pixel circuit 2 iscomposed of three transistors, one light emitting element, and onestorage capacitor. Note that in this third comparative example, thenumber of transistors is further reduced by one from that used in thesecond comparative example. The pixel circuit 2 includes a samplingtransistor Tr1, a storage capacitor Cs connected to the samplingtransistor Tr1, a driving transistor Trd connected to the storagecapacitor Cs, a light emitting element EL connected to the drivingtransistor Trd, and a switching transistor Tr4 forconnecting/disconnecting the driving transistor Trd to/from the powersupply Vcc.

The sampling transistor Tr1 turns on in response to the control signalWS supplied via the first scanning line WS and samples the image signalVsig supplied via the signal line SL at the storage capacitor Cs. Theimage signal Vsig sampled in the storage capacitor Cs is applied as aninput voltage Vgs to the gate G of the driving transistor Trd. Thedriving transistor Trd supplies an output current Ids corresponding tothe input voltage Vgs to the light emitting element EL. Note that theoutput current Ids depends on the threshold voltage Vth of the drivingtransistor Trd. The light emitting element EL is driven by the outputcurrent Ids supplied from the driving transistor Trd and emits lightwith intensity corresponding to the supplied image signal Vsig. Theswitching transistor Tr4 turns on in response to the control signal DSsupplied via the second scanning line DS to connect the drivingtransistor Trd to the power supply Vcc during the light emission period.In the non-light emission period, the switching transistor Tr4 turns offto electrically disconnect the driving transistor Trd from the powersupply Vcc.

A scanner unit including a write scanner 4 and a drive scanner 5 outputscontrol signals WS and DS over the first and second scanning lines WSand DS during each horizontal scanning period (1H) to properly turnon/off the sampling transistor Tr1 and the switching transistor Tr4 soas to perform the preliminary process, the correction process, and thesampling process. In the preliminary process, the storage capacitor Csis reset in preparation for the following correction process toeliminate the effects of the dependence of the output current Ids on thethreshold voltage Vth. In the correction process, a voltage foreliminating the effects of the threshold voltage Vth is written in thestorage capacitor Cs in the reset state. In the sampling process afterthe correction process, the signal potential of the image signal Vsig issampled into the storage capacitor Cs subjected to the correctionprocess. On the other hand, a signal unit including a horizontalselector (driver IC) 3 switches the potential of the image signal amonga fixed first potential VssH, second fixed potential VssL, and a signalpotential Vsig in each horizontal period (1H) thereby to provide, viathe signal line SL, various potentials for use in the preliminaryprocess, the correction process, and the sampling process.

More specifically, the horizontal selector 3 first supplies the firstfixed potential VssH with a high level and then switches the potentialto the second fixed potential VssL with a low level thereby allowing thepreliminary process to be performed. In the state in which the secondfixed low potential VssL is maintained, the preliminary process isperformed. Thereafter, the potential is switched to the signal potentialVsig, and the sampling process is performed. As described above, thehorizontal selector 3 is formed using the driving IC including a signalgenerator adapted to generate the signal potential Vsig, and an outputcircuit adapted to insert the first fixed potential VssH and the secondfixed potential VssL in the signal potential Vsig output from the signalgenerator thereby converting the image signal into a form in which thefirst fixed potential VssH, the second fixed potential VssL, and thesignal potential Vsig appear in turn, and output the resultant convertedimage signal over the respective signal lines SL.

The output current Ids of the driving transistor Trd depends not only onthe threshold voltage Vth but also on the carrier mobility μ of thechannel region of the driving transistor. Thus, the scanner unitincluding the write scanner 4 and the drive scanner 5 outputs thecontrol signal over the second scanning line DS in the horizontalscanning period (1H) to control the switching transistor Tr4 to allowthe correction process to be performed such that when the image signalVsig is being sampled, the output current Ids of the driving transistorTrd is extracted and negatively fed back to the storage capacitor Csthereby correcting the input voltage Vgs so as to delete the effects ofthe dependence of the output current Ids on the carrier mobility μ.

FIG. 12 is a timing chart provided for an explanation of the operationof the third comparative example of the pixel circuit shown in FIG. 11.Referring to FIG. 12, the operation of the pixel circuit shown in FIG.11 is described below. FIG. 12 illustrates waveforms, along time T, ofcontrol signals applied to the scanning lines WS and DS. For simplicity,the control signals are denoted by the same symbols as those denotingthe scanning lines. In FIG. 12, the waveform of the image signal Vsigapplied to the signal line is also shown along the time axis T. As shownin FIG. 12, the potential of the image signal is sequentially switchedamong the first fixed potential VssH, the second fixed potential VssL,and the signal potential Vsig. Because the transistor Tr1 is of theN-channel type, it turns on when the scanning line WS is at the highlevel while it turns off when the scanning line WS is at the low level.On the other hand, the P-channel type transistor Tr4 turns off when thescanning line DS is at the high level and turns on when the scanningline DS is at the low level. In the timing chart of FIG. 12, in additionto the waveforms of the control signals WS and DS and the waveform ofthe image signal, changes in voltages of the gate G and the source S ofthe driving transistor Trd are also shown.

In the timing chart shown in FIG. 12, a period from time T1 to time T8corresponds to one field (1 f) in which the rows of the pixel array aresequentially scanned once. Note that in the timing chart, the waveformsof the control signals WS and DS applied to pixels are shown only forone row.

First, at time T1, the switching transistor Tr4 is turned off such thatno light is emitted. In this state, no power is supplied from Vcc, andthus the source voltage of the driving transistor Trd falls down to thecut-off voltage VthEL of the light emitting element EL.

Thereafter, at time T2, the sampling transistor Tr1 is turned on. It isdesirable that before the sampling transistor Tr1 is turned on, thesignal line be raised to VssH, because this will reduce the writingtime. The turning-on of the sampling transistor Tr1 causes VssH to bewritten at the gate of the driving transistor Trd. In this process, thevoltage applied to the gate is coupled to the source via the storagecapacitor Cs, and thus the source potential rises up. However, therising of the potential of the source S is temporary. That is, thestorage capacitor Cs is discharged via the light emitting element EL andthe source potential falls down VthEL. During this transition, the gatevoltage remains at VssH.

Thereafter, at time Ta, the signal voltage is switched to VssL whilemaintaining the sampling transistor Tr1 in the on-state. This voltagechange is coupled to the source potential via the storage capacitor C.The amount coupling is given by Cs/(Cs+Coled)×(VssH−VssL) where the gatepotential is given by VssL and the source potential is given byVthEL−Cs/(Cs+Coled)×(VssH−VssL). Because of negative biasing, the sourcevoltage is lower than VthEL and thus the light emitting element EL turnsoff. Herein it is desirable that the source potential be set to a valuewhich causes the light emitting element EL to remain in the off-stateeven in the following Vth correction process and the mobility correctionprocess. By performing the coupling such that Vgs>Vth, it becomespossible to properly perform the Vth correction process in a followingperiod. Thus, in the present circuit configuration including a lessnumber of elements such as transistor, power supply lines, and gatelines, it is also possible to properly perform the preliminary processfor preparation for the Vth correction process. That is, the period fromT2 to Ta is included in the preliminary period for preparation for thecorrection process.

Thereafter, at time T3, the switching transistor Tr4 is turned on whilemaintaining the gate G at VssL. As a result, a current flows through thedriving transistor Trd, and the Vth correction process is performed in asimilar manner to the comparative examples. The current flows until thedriving transistor Trd turns off. If the driving transistor Trd turnsoff, the source potential of the driving transistor Trd becomes equal toVssL−Vth. Herein, it is necessary that VssL−Vth<VthEL is satisfied.

Thereafter, at time T4, the switching transistor Tr4 is turned off.Thus, the Vth correction process is completed. As described above, theperiod from T3 to T4 functions as the Vth correction period.

After the completion of the Vth correction process in the period fromtime T3 to time T4, at time T5, the potential of the signal line isswitched from VssL to Vsig. As a result, the signal potential of theimage signal Vsig is written in the storage capacitor Cs. Thecapacitance of the storage capacitor Cs is much smaller than thecapacitance of the equivalent capacitance Coled of the light emittingelement EL. Therefore, most of the signal potential Vsig is written inthe storage capacitor Cs. Thus, the voltage Vgs between the gate G andthe source S of the driving transistor Trd is given by Vth detected inthe previous step plus the sampled voltage Vsig, i.e., Vsig+Vth. Thatis, the input voltage Vgs equal to Vsig+Vth is applied to the drivingtransistor Trd. The sampling of the signal voltage Vsig is continueduntil the control signal WS is returned to the low level at time T7.Thus, the period from T5 to T7 functions as the sampling period.

In the present comparative example of the pixel circuit, in addition tothe Vth correction process, the correction process associated with themobility μ is also performed. The correction associated with themobility μ is performed in the period from T6 to T7 As shown in thetiming chart, a correction voltage ΔV is subtracted from the inputvoltage Vgs.

At time T7, the control signal WS falls down to the low level, and thusthe sampling transistor Tr1 turns off. As a result, the gate G of thedriving transistor Trd is disconnected from the signal line SL. Thus,the image signal Vsig is no longer applied to the gate G of the drivingtransistor Trd, and the gate potential (G) of the driving transistor Trdrises up with the source potential (S). In this transition process, thegate-source voltage Vgs is maintained at the voltage stored in thestorage capacitor Cs, i.e., (Vsig−ΔV+Vth). As a result of the increasein the source potential (S), the bias of the light emitting element ELis switched from the reverse bias to the forward bias, and the outputcurrent Ids flows into the light emitting element EL. Thus, the lightemitting element EL starts emitting light.

Finally, at time T8, the control signal DS rises up to the high level,and the switching transistor Tr4 turns off. As a result, light emissionis stopped, and the present field ends. Thereafter, a next field issubjected to the similar operation including the preliminary process,the Vth correction process, the mobility correction process, and thelight emission process.

FIG. 13 is a circuit diagram of a display apparatus according to a thirdembodiment of the present invention. This display apparatus isconfigured in a basically similar manner to the third comparativeexample shown in FIG. 11. In FIG. 13, similar parts to those in FIG. 11are denoted by similar reference symbols. The display apparatusaccording to the third embodiment of the invention is different from thethird comparative example in that an additional switching transistor Tr6is disposed between the driving transistor Trd and the light emittingelement EL. Furthermore, the equivalent capacitor Coled of the lightemitting element EL is removed, and an auxiliary capacitor Csub isconnected between the source S of the driving transistor Trd and aground line. To control turning-on/off of the gate of the switchingtransistor Tr6, additional scanning lines DS2 are disposed. The scanninglines DS2 are sequentially scanned line by line by a second drivescanner 8 additionally disposed in a driving unit.

FIG. 14 is a timing chart provided for an explanation of the operationof the pixel circuit according to the third embodiment shown in FIG. 13.This timing chart is basically similar to that shown in FIG. 12 andsimilar parts to those in FIG. 12 are denoted by similar referencesymbols. As shown in FIG. 14, at time T1, the switching transistor Tr4is turned off, and thus the non-light emission period starts. At time T1a, the switching transistor Tr6 is turned off thereby electricallydisconnecting the light emitting element EL from the driving transistorTrd. Note that the turning-off of the switching transistors Tr4 and Tr6may be performed in the reverse order. Thereafter, the threshold voltagecorrection process, the image signal sampling process, and the mobilitycorrection process are sequentially performed. Then, at time T7, thesampling transistor Tr1 is turned off to disconnect the gate G of thedriving transistor Trd from the signal line SL. Thus, the preliminaryprocess, the correction processes, and the sampling process are allcompleted, and it is now ready to perform the light emission process.Note that in the operation described above, the switching transistor Tr6is maintained in the off-state, and the light emitting element EL isdisconnected from the source S of the driving transistor Trd so that theleakage current of the light emitting element EL does not have adverseeffects on the operation.

Thereafter, at time T7 a, the switching transistor Tr6 again turns on,and thus the light emitting element EL is electrically connected to thedriving transistor Trd. As a result, the driving current Ids flows fromthe power supply Vcc into the cathode via the light emitting element EL,and thus light emission starts. In the above process, the potential ofthe source S of the driving transistor Trd falls down from the powersupply potential Vcc to the operating point of the light emittingelement EL and settles there.

FIG. 15 is a circuit diagram of a fourth comparative example of adisplay apparatus. In this fourth comparative example, each pixelcircuit includes two transistors, one light emitting element, and onestorage capacitor. Compared with the third comparative example describedabove, a further reduction in the number of transistors is achieved. Asshown in FIG. 15, the pixel 2 includes a light emitting element EL suchas an organic electroluminescent element, a sampling transistor Tr1, adriving transistor Trd, and a storage capacitor Cs. The control terminal(the gate) of the sampling transistor Tr1 is connected to acorresponding scanning line WS. One of current terminals (the source andthe drain) of the sampling transistor Tr1 is connected to acorresponding signal line SL, and other one is connected to the controlterminal (the gate) of the driving transistor Trd. One of currentterminals (the source and the drain) of the driving transistor Trd isconnected to the light emitting element EL, and the other one isconnected to a corresponding power supply line VL. More specifically, inthe present example, the driving transistor Trd is of the N-channeltype, and thus the drain thereof is connected to the power supply lineVL and the source S serving as an output node is connected to the anodeof the light emitting element EL. The cathode of the light emittingelement EL is connected to a cathode potential Vcath. The storagecapacitor Cs is connected between the source S and the gate G of thedriving transistor Trd.

In this configuration, the sampling transistor Tr1 turns on in responseto the control signal supplied via the scanning line WS. As a result,the signal potential supplied via the signal line SL is sampled andstored in the storage capacitor Cs. The driving transistor Trd issupplied with a current from the power supply line VL at a firstpotential (high potential Vcc) and supplies a driving current, dependingon the signal potential stored in the storage capacitor Cs, to the lightemitting element EL. The write scanner 4 outputs a control signal with apredetermined pulse width over the control line WS thereby to turn onthe sampling transistor Tr1 over a period in which the signal line SL isat the signal potential, whereby the signal potential is stored in thestorage capacitor Cs, and the correction voltage for eliminating theeffects of the mobility μ of the driving transistor Trd is added to thesignal potential. Thereafter, the driving transistor Trd supplies thedriving current, corresponding to the signal potential Vsig stored inthe storage capacitor Cs, to the light emitting element EL, and thus thelight emission starts.

In addition to the mobility correction function, the pixel circuit 2also has a threshold voltage correction function. More specifically, ata first point of time before the sampling transistor Tr1 performs thesampling of the signal potential Vsig, a power scanner 6 switches thepotential of the power supply line VL from the first potential (highpotential Vcc) to a second potential (low potential) Vss. At a secondpoint of time before the sampling transistor Tr1 performs the samplingof the signal potential Vsig, the write scanner 4 turns on the samplingtransistor Tr1 whereby a reference potential Vref is applied from thesignal line SL to the gate G of the driving transistor Trd and thesource S of the driving transistor Trd is set at the second potential(Vss). At a third point of time after the second point of time, thepower scanner 6 switches the potential of the power supply line VL fromthe second potential Vss to the first potential Vcc whereby a voltagecorresponding to the threshold voltage Vth of the driving transistor Trdis stored in the storage capacitor Cs. As a result of the thresholdvoltage correction process described above, the effects of thepixel-to-pixel variation in the threshold voltage Vth of the drivingtransistor Trd are eliminated.

The pixel circuit 2 also has a bootstrap function. More specifically, ata time at which the signal potential Vsig has been stored in the storagecapacitor Cs, the write scanner 4 removes the application of the controlsignal to the scanning line WS thereby to turn off the samplingtransistor Tr1 and electrically disconnect the gate G of the drivingtransistor Trd from the signal line SL whereby the potential of the gateG of the driving transistor Trd changes with the change in potential ofthe source S and thus the voltage Vgs between the gate G and the sourceS is maintained constant.

FIG. 16 is a timing chart provided for an explanation of the operationof the fourth comparative example of the pixel circuit shown in FIG. 15.In FIG. 16, changes in potentials of the scanning line WS, the powersupply line VL, and the signal line SL along a common time axis areshown. In addition to these potential changes, changes in potentials ofthe gate G and the source S of the driving transistor are also shown. Asdescribed above, the control pulse signal for turning on the samplingtransistor Tr1 is applied to the scanning line WS. This control pulsesignal is periodically applied to the scanning line WS at a repetitioninterval corresponding to one field (1 f) in synchronization with theline-by-line scanning of the pixel array. The potential of the powersupply line VL is periodically switched between the high potential Vccand the low potential Vss at the interval corresponding to one field. Animage signal is supplied to the signal line SL such that the potentialof image signal is switched between the signal potential Vsig and thereference potential Vref in each horizontal period (1H).

As shown in the timing chart of FIG. 16, after the end of a lightemission period in a previous field, a non-light emission period in apresent field starts. After the end of the non-light emission period, alight emission period in the present field starts. In the non-lightemission period, the preliminary process, the threshold voltagecorrection process, the signal writing process, and the mobilitycorrection process are performed.

In the light emission period in the previous field, the power supplyline VL was at the high voltage Vcc and the driving current Ids wassupplied from the driving transistor Trd to the light emitting elementEL such that the driving current Ids flowed from the power supply lineVL at the high potential Vcc into light emitting element EL via thedriving transistor Trd and further into the cathode line.

At the start of the non-light emission period in the present field,first, at time T1, the potential of the power supply line VL is switchedfrom the high potential Vcc to the low potential Vss such that thepotential of the power supply line VL falls down to Vss and thus thepotential of the source S of the driving transistor Trd falls down toVss. As a result, the anode of the light emitting element EL (i.e., thesource of the driving transistor Trd) is brought at a potential whichcauses the light emitting element EL to be reversely biased, and thus nodriving current flows through the light emitting element EL and lightemission stops. Furthermore, as the potential of the source S of thedriving transistor falls down, the potential of the gate G falls down.

Thereafter, at time T2, the scanning line WS is switched from the lowlevel to the high level thereby to turn on the sampling transistor Tr1.At this point of time, the signal line SL is at the reference potentialVref. As a result, the gate G of the driving transistor Trd is broughtat the reference potential Vref of the signal line SL via the turned-onsampling transistor Tr1. In this state, the source S of the drivingtransistor Trd at the potential Vss that is sufficiently lower thanVref. Thus, the voltage Vgs between the gate G and the source S of thedriving transistor Trd is reset to the value greater than the thresholdvoltage Vth of the driving transistor Trd. Thus, the period from T1 toT3 functions as the preliminary period in which the gate-source voltageVgs of the driving transistor Trd is reset to the value greater thanVth.

Thereafter, at time T3, the power supply line VL is switched from thelow potential Vss to the high potential Vcc and thus the potential ofthe source S of the driving transistor Trd starts to rise up. When thegate-source voltage Vgs of the driving transistor Trd reaches a valueequal to the threshold voltage Vth, the driving transistor Trd turnsoff. Thus, the voltage corresponding to the threshold voltage Vth of thedriving transistor Trd is written in the storage capacitor Cs therebyperforming the correction associated with the threshold voltage. In thisstate, in order that all current flows into the storage capacitor Cs andno current flows through the light emitting element EL, the cathodepotential Vcath is set to a value that causes the light emitting elementEL to be in the off-state. The threshold voltage correction processdescribed above is completed by time T4 at which the signal line SL isswitched from Vref to Vsig. Thus, the period from T3 to T4 functions asthe threshold voltage correction period.

At time T4, the signal line SL is switched from the reference potentialVref to the signal potential Vsig. At point of time, the samplingtransistor Tr1 is still maintained in the on-state. Thus, the gate G ofthe driving transistor Trd is brought to the signal potential Vsig. Inthis state, because the light emitting element EL is in the cut-offstate in which the light emitting element EL has a high impedance, thecurrent flowing between the drain and source of the driving transistorTrd is all supplied into the storage capacitor Cs and the equivalentcapacitance of the light emitting element EL, and thus the chargingprocess starts. The potential of the source S of the driving transistorTrd rises up by KV by time T5 at which the sampling transistor Tr1 turnsoff. As a result, the signal potential Vsig of the image signal is addedto Vth and the resultant total value is stored in the storage capacitorCs and furthermore the voltage ΔV for the mobility correction issubtracted from the voltage stored in the storage capacitor Cs. Thus,the period from time T4 to time T5 functions as the signalwriting/mobility correction period. In the signal writing period from T4to T5, as described above, the writing of the signal potential Vsig andthe adjustment of the correction amount ΔV are performed concurrently.The higher the signal potential Vsig, the greater the current Idssupplied by the driving transistor Trd and the greater the absolutevalue of ΔV. Thus, the mobility correction is performed properlydepending on the light emission intensity. For the same signal potentialVsig, the greater the mobility μ of the driving transistor Trd, thegreater the absolute value of ΔV. In other words, the greater themobility μ, the greater the amount ΔV of the negative feedback to thestorage capacitor Cs, and thus the effects of the pixel-to-pixelvariation in the mobility μ are eliminated.

Finally, at time T5, the potential of the scanning line WS is switchedto the low level, and thus the sampling transistor Tr1 turns off. As aresult, the gate G of the driving transistor Trd is disconnected fromthe signal line SL, and the drain current Ids starts to flow into thelight emitting element EL. Thus, the anode potential of the lightemitting element EL rises up depending on the driving current Ids. Therising of the anode potential of the light emitting element EL meansthat the potential of the source S of driving transistor Trd also risesup. As the potential of the source S of the driving transistor Trd, thepotential of the gate G of the driving transistor Trd also rises up viathe bootstrap operation of the storage capacitor Cs by the same amountas the increase in the source potential. Thus, the gate-source voltageVgs of the driving transistor Trd is maintained constant during thelight emission period. The value of Vgs is given by the sum of thesignal potential Vsig and the threshold voltage Vth and the correctionvalue associated with the mobility μ.

FIG. 17 is a circuit diagram of a display apparatus according to afourth embodiment of the present invention. This display apparatus isconfigured in a basically similar manner to the fourth comparativeexample shown in FIG. 15. In FIG. 17, similar parts to those in FIG. 15are denoted by similar reference symbols. The display apparatusaccording to the fourth embodiment of the invention is different fromthe fourth comparative example shown in FIG. 15 in that a switchingtransistor Tr6 is disposed between the source S of the drivingtransistor Trd and the anode of the light emitting element EL, and anauxiliary capacitor Csub is connected between the source S of thedriving transistor Trd and a ground line. To drive the gate of theswitching transistor Tr6, additional scanning lines DS extendingparallel to the scanning lines WS are disposed. These additionalscanning lines DS are sequentially scanned line by line by a drivescanner 8 disposed in a driving unit.

FIG. 18 is a timing chart provided for an explanation of the operationof the pixel circuit according to the fourth embodiment shown in FIG.17. This timing chart is basically similar to that shown in FIG. 16 andsimilar parts to those in FIG. 16 are denoted by similar referencesymbols. First, at time T1 a in a light emission period, the switchingtransistor Tr6 is turned off. As a result, the driving transistor Trd iselectrically disconnected from the light emitting element EL, and anon-light emission period starts. At this point of time, the drivingtransistor Trd is still in the on-state, and thus the potential of thesource S is pulled up to Vcc by the power supply line. As a result, thepotential of the gate G of the driving transistor Trd also rises up.

Thereafter, in the state in which the light emitting element EL iselectrically disconnected from the driving transistor Trd, thepreliminary process for preparation for the correction, the thresholdvoltage correction process, the signal writing process, and the mobilitycorrection process are performed. More specifically, at time T1, thepower supply line VL is switched from Vcc to Vss to set the sourcepotential of the driving transistor Trd to Vss. Furthermore, at time T2,the sampling transistor Tr1 is turned on while maintaining the signalline SL at the reference voltage Vref such that the gate potential ofthe driving transistor Trd is set to Vref. Thus, the preliminary processfor preparation for the threshold voltage correction is completed.Thereafter, at time T3, the power supply line VL is switched from Vss toVcc, and the storage capacitor Cs is charged until the drivingtransistor Trd turns off. As a result, the threshold voltage Vth iswritten in the storage capacitor Cs. Thereafter, at time T4, the signalline SL is switched to the signal potential Vsig, and the signalpotential Vsig is written in the storage capacitor Cs. The writingprocess is continued until time T5. At time T5, the sampling transistorTr1 is turned off. In the period from time T4 to time T5, in addition tothe writing of the signal potential Vsig, the correction process isperformed to eliminate the effects of the variation in the mobility μ.At time T5, the sampling transistor Tr1 turns off, and the gate G of thedriving transistor Trd is electrically disconnected from the signal lineSL. Thus, the pixel circuit is in a state in which light can be emitted.In this state, because the driving transistor Trd is electricallydisconnected from the light emitting element EL, the source potential ofthe driving transistor Trd is pulled up to the high potential Vccsupplied via the power supply line VL.

If the sampling transistor Tr1 turns off, then at time T5 a, theswitching transistor Tr6 is turned on to electrically connect the lightemitting element EL to the driving transistor Trd. As a result, thedriving current Ids flows from the power supply line VL into the lightemitting element EL via the driving transistor Trd, and thus a lightemission period starts.

FIG. 19 illustrates an example of a structure of a thin-film device usedin the display apparatus according to the present invention. In thisfigure, a cross-sectional structure of a pixel formed on an insulatingsubstrate is schematically shown. As shown in this figure, the pixelincludes a plurality of thin-film transistors (only one thin-filmtransistor is shown in FIG. 19), a storage capacitor, and a lightemission element such as an organic electroluminescent element. Thetransistors and the capacitor are formed in a lower layer on thesubstrate by using a TFT process, and the light emitting element (theorganic electroluminescent element) is formed in an upper layer. Atransparent opposite substrate is bonded thereto via an adhesive therebyforming a flat panel.

The display apparatus according to the present invention may be realizedin a flat module form as shown in FIG. 20. More specifically, forexample, a pixel array including a plurality of pixels each having anorganic electroluminescent element, thin-film transistors, a thin-filmcapacitor is formed on an insulating substrate, and an oppositesubstrate made of glass or the like is bonded to the insulatingsubstrate via an adhesive disposed around the pixel array therebyforming a display module. As required, additional parts such as a colorfilter, a protective film, a light blocking film, etc. may be disposedon the transparent opposite substrate. An FPC (flexible printed circuit)for a connection between the pixel array and an external circuit may bedisposed on the display module.

The display apparatus in the form of a flat panel according to thepresent invention may be used as a display for displaying a still ormoving image in various kinds of electronic devices such as a digitalcamera, a notebook personal computer, a portable telephone, or a videocamera. Some specific examples of such electronic devices are shownbelow.

FIG. 21 illustrates a television set using an image display screen 11including a front panel 12 and filter glass 13. The display apparatusaccording to the present invention may be used as the image displayscreen 11.

FIG. 22 illustrates a digital camera. A front view thereof is shown onthe top of the figure, and a rear view is shown on the bottom. Thedigital camera includes an imaging lens, a flash unit 15, a display 16,a control switch, a menu switch, and a shutter button 19. The displayapparatus according to the present invention may be used as the display16.

FIG. 23 illustrates a notebook personal computer. A main body 20 thereofincludes a keyboard 21 operated by a user to input data or a command. Acover includes a display 22 for displaying image. The display apparatusaccording to the present invention may be used as the display 22.

FIG. 24 illustrates a portable terminal. The portable terminal in anopen state is shown on the left-hand side of the figure, while theportable terminal in a closed state is shown in the right-hand side. Theportable terminal includes an upper case 23, a lower case 24, aconnecting part (hinge) 25, a display 26, a sub-display 27, a picturelight 28, and a camera 29. The display apparatus according to thepresent invention may be used as the display 26 or the sub-display 27.

FIG. 25 illustrates a video camera including a main body 30, an imaginglens 34 disposed on a front surface, a start/stop switch 35, and amonitor 36. The display apparatus according to the present invention maybe used as the monitor 36.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A display apparatus comprising: a plurality ofpixels; and a driving unit adapted to drive the plurality of pixels,each of the plurality of pixels including at least a samplingtransistor, a driving transistor, a light emitting element, a firststorage capacitor, a second storage capacitor, a first switchingtransistor, and a second switching transistor the driving unit beingconfigured to output a control signal to turn on the samplingtransistor, and output an image signal to write the image signal in thefirst storage capacitor via the turned-on sampling transistor, thedriving transistor being configured to supply a driving current,depending on a signal voltage of the written image signal, to the lightemitting element during a predetermined light emission period, the firstswitching transistor being connected between a voltage line and thedriving transistor, the second switching transistor being connectedbetween the driving transistor and the light emitting element, thesecond switching transistor being adapted to turn off during a non-lightemission period.
 2. The display apparatus according to claim 1, whereina gate of the driving transistor is connected to a first referencepotential, and a source of the driving transistor is connected to asecond reference potential, and a threshold voltage of the drivingtransistor Vth, the first reference potential Vss1, and the secondreference potential Vss2 are set to satisfy the following formulaVss1−Vss2>=Vth.
 3. The display apparatus according to claim 2, whereinthe threshold voltage of the light emitting element EL is denoted byVthEL, and parameters are set such that VthEL>Vss2 is satisfied.
 4. Thedisplay apparatus according to claim 1, further including scanning lineseach connected to the respective sampling transistors of ones of theplurality of pixels, and additional scanning lines extending parallel tothe scanning lines.
 5. The display apparatus according to claim 4,wherein the additional scanning lines are sequentially scanned line byline.
 6. The display apparatus according to claim 1, wherein the secondstorage capacitor is connected between the source of the drivingtransistor and a ground line.
 7. The display apparatus according toclaim 1, wherein the transistors and the capacitor are formed on thesubstrate, and the light emitting element is formed on the transistorsand the capacitor.
 8. The display apparatus according to claim 7,wherein an opposite substrate is disposed on the light emitting element,and the opposite substrate is bonded to the substrate via an adhesive.9. The display apparatus according to claim 8, wherein a color filter isdisposed on the opposite substrate.
 10. The display apparatus accordingto claim 1, wherein the second switching transistor, when turned off,prevents a current from the driving transistor to the light emittingelement while a correction voltage is written in the first storagecapacitor.
 11. An electronic device comprising: a display panelincluding: a plurality of pixels, and a driving unit adapted to drivethe plurality of pixels; at least one circuit board that is notintegrated with the display panel; and at least one flexible printedcircuit that connects the driving unit to the at least one circuit boardand through which an image signal is input to the driving unit, each ofthe plurality of pixels including at least a sampling transistor, adriving transistor, a light emitting element, a first storage capacitor,a second storage capacitor, a first switching transistor, and a secondswitching transistor the driving unit being configured to output acontrol signal to turn on the sampling transistor, and output an imagesignal to write the image signal in the first storage capacitor via theturned-on sampling transistor, the driving transistor being configuredto supply a driving current, depending on a signal voltage of thewritten image signal, to the light emitting element during apredetermined light emission period, the first switching transistorbeing connected between a voltage line and the driving transistor, thesecond switching transistor being connected between the drivingtransistor and the light emitting element, the second switchingtransistor being adapted to turn off during a non-light emission period.12. A display apparatus comprising: a plurality of pixels; and a drivingunit adapted to drive the plurality of pixels, each of the plurality ofpixels including at least a sampling transistor, a driving transistor, alight emitting element, a first storage capacitor, a second storagecapacitor, a first switching transistor, and a second switchingtransistor, the driving unit being configured to output a control signalto turn on the sampling transistor, and output an image signal to writethe image signal in the first storage capacitor via the turned-onsampling transistor, the driving transistor being configured to supply acorrection current to the first storage capacitor and the second storagecapacitor during a correction period before a light emission period, andsupply a emission current to the light emitting element during the lightemission period, the first switching transistor being connected betweena voltage line and the driving transistor, the second switchingtransistor being connected between the driving transistor and the lightemitting element, the second switching transistor being adapted to turnoff during the correction period.
 13. The display apparatus according toclaim 12, wherein a gate of the driving transistor is connected to afirst reference potential, and a source of the driving transistor isconnected to a second reference potential, and a threshold voltage ofthe driving transistor Vth, the first reference potential Vss1, and thesecond reference potential Vss2 are set to satisfy the following formulaVss1−Vss2>=Vth.
 14. The display apparatus according to claim 13, whereinthe threshold voltage of the light emitting element EL is denoted byVthEL, and parameters are set such that VthEL>Vss2 is satisfied.
 15. Thedisplay apparatus according to claim 12, wherein further includingscanning lines each connected to the respective sampling transistors ofones of the plurality of pixels, and additional scanning lines extendingparallel to the scanning lines.
 16. The display apparatus according toclaim 15, wherein the additional scanning lines are sequentially scannedline by line.
 17. The display apparatus according to claim 12, whereinthe second storage capacitor is connected between the source of thedriving transistor and a ground line.
 18. The display apparatusaccording to claim 12, wherein the transistors and the capacitor areformed on the substrate, and the light emitting element is formed on thetransistors and the capacitor.
 19. The display apparatus according toclaim 18, wherein an opposite substrate is disposed on the lightemitting element, and the opposite substrate is bonded to the substratevia an adhesive.
 20. The display apparatus according to claim 19,wherein a color filter is disposed on the opposite substrate.
 21. Thedisplay apparatus according to claim 12, wherein the second switchingtransistor, when turned off, prevents a current from the drivingtransistor to the light emitting element while a correction voltage iswritten in the storage capacitor.
 22. An electronic device comprising: adisplay panel including: a plurality of pixels, and a driving unitadapted to drive the plurality of pixels; at least one circuit boardthat is not integrated with the display panel; and at least one flexibleprinted circuit that connects the driving unit to the at least onecircuit board and through which an image signal is input to the drivingunit, each of the plurality of pixels including at least a samplingtransistor, a driving transistor, a light emitting element, a firststorage capacitor, a second storage capacitor, a first switchingtransistor, and a second switching transistor, the driving unit beingconfigured to output a control signal to turn on the samplingtransistor, and output an image signal to write the image signal in thefirst storage capacitor via the turned-on sampling transistor, thedriving transistor being configured to supply a correction current tothe first storage capacitor and the second storage capacitor during acorrection period before a light emission period, and supply a emissioncurrent to the light emitting element during the light emission period,the first switching transistor being connected between a voltage lineand the driving transistor, the second switching transistor beingconnected between the driving transistor and the light emitting element,the second switching transistor being adapted to turn off during thecorrection period.